■fl 472 
,F75 
Jopy 1 



DEPARTMENT OF COMMERCE 



Technologic Papers 



OP THE 



Bureau of Standards 

S. W. STRATTON. Director 



No. 219 

( Part of Vol. 16] 

EFFECT OF TEMPERATURE, DEFORMATION, AND 
RATE OF LOADING ON THE TENSILE PROP- 
ERTIES OF LOW-CARBON STEEL BELOW 
THE THERMAL CRITICAL RANGE 

BY 

H. J. FRENCH, Physicist 
Bureau of Standards 



AUGUST 22, 1922 




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1922 



DEPARTMENT OF COMMERCE 



Technologic Papers 



OF THE 



Bureau of Standards 

S. W. STRATTON, Director 



No. 219 

[ Part ol Vol. 16] 

EFFECT OF TEMPERATURE, DEFORMATION, AND 
RATE OF LOADING ON THE TENSILE PROP- 
ERTIES OF LOW-CARBON STEEL BELOW 
THE THERMAL CRITICAL RANGE 

BY 

H. J. FRENCH, Physicist 

Bureau of Standards 



AUGUST 22, 1922 




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$1.25 PER VOLUME ON SUBSCRIPTION 

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Washington, D. C. 

WASHINGTON 
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1922 



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LIBRARY OF CONGRESS ' i 
J?ECE1VED 

NOV 231922 

DOCUWI&NTS DIVISION 



:^^-2i^cg 



EFFECT OF TEMPERATURE, DEFORMATION, AND 
RATE OF LOADING ON THE TENSILE PROPERTIES 
OF LOW-CARBON STEEL BELOW THE THERMAL 
CRITICAL RANGE 

By H. J. French 



ABSTRACT 

An apparatus for determining tensile properties of metals at high temperatures 
(including limit of proportionality) and the results of tests of several grades of boiler 
plate from 20 to 465° C. are described. The effects of cold and blue work on the 
properties of these steels throughout the range given are discussed, and in addition, 
some results are given showing the effect of tensional elastic overstrain on the pro- 
portional limit at different temperatures and its subsequent behavior with time. 
Effects of variations in rates of loading (both rapid and slow) and the modified 
apparatus used for this work are also described. 



CONTENTS 

Page 

I. Introduction 680 

II. Tensile properties of steels at high temperatures 681 

1. Previous investigations 68r 

2. Materials and methods used 685 

(a) Steels tested 685 

(6) Apparatus for determination of proportional limit 686 

(c) Heating furnace 688 

(d) Test procedure 688 

(e) Thermal equilibrium 689 

3. Tensile properties of hot-rolled boiler plate at elevated tempera- 

tures 691 

III. Effect of permanent cold and blue deformation on the tensile properties of 

steel at various temperatures 693 

1. Cold-rolling 693 

2 . Blue-rolling 697 

3. Depth of penetration of effect of blue and cold-rolling 703 

4. Tensile tests of transverse specimens of hot, cold, and blue rolled 

boiler plate at various temperatures 704 

5. Permanent deformation produced by stretching 706 

IV. Effect of rate of loading on the tensile properties of steel at various tem- 

peratures 712 

1. Rapid loading 7 14 

(a) Apparatus used 714 

(b) Experimental results 718 

2. Slow loading 718 

V. Microscopic examination 719 

679 



68o Technologic Papers of the Bureau of Standards [Voi. i6 

Page. 
VI. Discussion and summary 719 

1 . Amorphous metal theory 719 

2. Summary 721 

3. Acknowledgements 723 

VII. Selected bibliography 724 

1. Properties of ferrous alloys at elevated temperatures 724 

2. Additional references 725 

(a) Cold and blue work 725 

(6) Elastic overstrain 725 

(c) Rate of loading 725 

I. INTRODUCTION 

During 1 91 9 the Bureau of Standards was requested by a com- 
mittee of the engineering division/ National Research Council, 
to determine the effects of cold work on the proportional limit of 
boiler plate at elevated temperatures, as a result of a similar 
request received by this committee. Following the early work of 
development of a suitable apparatus for such tests about July, 
1 91 9, and the determination of the tensile properties of the hot- 
rolled steel for a basis of comparison, there arose the questions of 
deformation in the "blue-heat range" and the effects of varia- 
tions in rate of stress application on the properties of steel at high 
temperatures, so that the original program was greatly extended. 

The complete research necessitated investigation along five 
different lines, which may be summarized as follows: 

1. Development of apparatus for determination of the limit 
of proportionality at various temperatiures, and investigation of 
the tensile properties of several grades of boiler plate up to about 
465° C (870° F). 

2. Effect of permanent deformation (by rolling cold and at 
blue heat) on the properties of steel at different temperatures. 

3. Study of the behavior of steel subjected to tensional elastic 
overstrain at several temper atiu-es. 

4. Effect of rate of stress application on the tensile properties 
of hot-rolled boiler plate at various temperatures. 

5. Microscopic examination. 

As progress was made in this work new questions arose, but 
because of the almost luilimited field for investigation it was 
not possible to follow more than a few of such leads or to satisfy 
these inquiries. The tests are not considered complete in any of 
the several phases dealt with, but are presented as detailed 
data relative to several questions concerning which there have 

1 Committee on Physical Changes in Iron and Steel below the Thermal Critical Range. Dr. Zay 
JefEries, chairman. 



French] Steel at High Temperatures 68 1 

been conflicting opinions and likewise show effects, which, as far 
as the author is aware, have not before been so completely deter- 
mined quantitatively. In some cases the practical applications 
of results of tests are indicated, while at the conclusion of the 
report is given a selected bibliography on the mechanical prop- 
erties of steels at high temperatures, which will be referred to 
throughout the text as required. 

II. TENSILE PROPERTIES OF STEELS AT HIGH 
TEMPERATURES 

1. PREVIOUS INVESTIGATIONS 

It has long been known that increase in temperature above 
the ordinary atmospheric range is accompanied by changes in 
steels, particularly in strength and ductility. A large number of 
interesting and important papers dealing with various phases of 
this subject have appeared from time to time, but as recently 
pointed out by Jeffries (34) ^ our knowledge is still imsatisfactory, 
and a better understanding of these changes will undoubtedly be 
of benefit to industry. White (35) is of the opinion that our 
knowledge of the tensile properties of steels at high temperatiu-es is 
wholly inadequate and has not kept pace with advancement of 
knowledge in other branches of engineering, while Howe (38) long 
ago called attention to the apparent anomalies found in study- 
ing the effects of work at temperatures imder the thermal trans- 
formations on the properties of wrought iron and steel used in 
boiler construction. Certainly, from the standpoint of engineering 
design, it is important to know the variations in limit of propor- 
tionality with changes in temperature, but unfortunately this is the 
more difficult to determine of those factors considered in tensile 
tests, and the data available are conflicting. 

In Figures lA and iB is given a summary of results obtained 
in some of the more important published investigations of the 
tensile properties of steels and wrought iron at high tempera- 
tures. ^ Increase in strength with first rise in temperature above 
that of the room is reported by Rudeloff (9) , (13) , Himtington (24) , 
Kpps and Jones (32), and Bregowsky and Spring (25), while 
Martens (7), Perrine and Spencer (30), and J. B. Howard (8) 
find a slight decrease between about 50 and 150° C (125 and 

' These figures relate to the numbered references in the "Selected bibliography" at the conclusion o{ 
this paper. 
' Refer to the bibliography at the conclusion of this report for more complete data. 



682 



Technologic Papers of the Bureau of Standards [Voi. i6 



300° F), which is followed by increase to a maximum between 205 
and 345° C (400 and 600° F). In a very complete series of tests 
of steels of varying carbon contents Howard further f omid that the 
minimum tensile strength occurring with first rise in temperatiire 
was generally more quickly reached the lower the carbon content, 



Elon^afioti (in lindiei unless cthemist ^Mifitc^ 




200 30O MOO 

Degrees C 



Fig. iA.- 



-Tensile properties of wrought iron and, steels at various temperatures as 
determined by different investigators 

A Martens (1890), "Hartestufe" 

B Rudeloff (1893), Martin Stahl 

C Huntington, mild steel 

Ci Huntington, ■wrougiit iron 

D Bregowsky & Spring, cold-rolled Bessemer shafting 

El Perrine & Spencer, 0.23 per cent C, Bessemer steel 

£2 Perrine & Spencer, 0.39 per cent C, Bessemer steel 

F Epps and Jones, wrought iron 

G Shelby Laboratory, 0.36 per cent C steel 

and that the higher carbon steels attained their maximum strength 
at lower temperattires than medium or low-carbon alloys (Fig. 2). 
Various investigators have from time to time reported decrease 
in elastic properties with increase in temperatiire, but one of the 
earlier investigators, Martens (7), has distinguished between the 
gradual decrease in yield point and the behavior of the propor- 



French] 



Steel at High Temperatures 



683 



tional limit which, after a sHght decrease at about 100° C (210° 
F) in "soft steel," increased to a maximum at 200° C (390° F) 
before final decrease occurred. Kpps and Jones obtained similar 
inflections in a proportional limit temperature curve for wrought 



T msile Streng th 



m> 



eo 



20 



1 
I 

S 100 

g. 
I SO 



to 





zoo 

L_ 



6C0 mo 

Beirees F 



30O AOO 

Degrrts C 



600 



ioo 

L_ 



^000 BOO 

Deartes F 



2O0 300 *00 

Degrees C 



600 



Elastic Properties 



•j^dlMt., 




1 


1 


t 1 1 






1 


zoo 

1 


Degrees P 
1 1 1 


1000 
1 1 





100 


ZOO 300 AOO 


!ai ioo 




Degrees C 

Fig. iB. — Tensile properties of wrought iron and steels at various temperatures as 

determined by different investigators 

A Martens (1890), "Hartestufe" 

B Rudeloff (1893), Martin Stahl 

C Huntington, mild steel 

Ci Huntington, -wrought iron 

D Bregowsky & Spring, cold-rolled Bessemer shafting 

£1 Perrine & Spencer, 0.23 per cent C, Bessemer steel 

E2 Perrine & Spencer, 0.39 per cent C, Bessemer steel 

F Epps and Jones, wrought iron 

G Shelby lyaboratory, 0.36 per cent C Steel 

iron with first decrease at a slightly higher temperature and the 
maximum at about i8o° C (360° F), while both the Shelby labora- 
tory * and Bregowsky and Spring (25) report direct increase in 
yield point to a maximum at about 200° C (390° F) . 



* Private communication from Luken's Steel Co., 1920. 



684 



Technologic Papers of the Bureau of Standards [Voi. i6 



In his tests Howard (8) found that the interval between the 
elastic limit and the maximum stress showed particularly inter- 
esting features. Several of the different steels tested showed a 
yield point at the elastic limit, this period being marked by rapid 
stretching which, once begun, continued under reduced loads. 
Such yielding rarely occiurs in testing steel at room temperature, 
but was observed by Howard up to about 260° C (500° F) in 
tests of mild steels and at temperatmres somewhat below this in 
the higher carbon alloys. Bars tested between about 95 and 205° 
C (200 and 400° F) showed alternate periods of relaxation and 
rigidity tmder increasing stress resembling a succession of yield 
points, apparently indicating some remarkable changes taking 
place within the metal in this temperature range. 



Jkgreti C 




Jlegrves P 

FlG. 2. — Tensile strength of carbon steels at variotis temperatures as determined by 

J. E. Howard 

See Physical properties of iron and steel at higher temperature. Iron Age, 45. p. 585; 1890 

There is more general agreement in various publications regard- 
ing changes in ductility as measured by elongation and reduction 
of area. Elongation 'decreases slowly just above room tempera- 
ture and thereafter more rapidly to a minimum variously reported 
as occurring between 125 and about 200° C (255 and 390° F). 
It then increases rapidly. Reduction of area likewise decreases 
but little just above the temperature of the room, but then drops 
rapidly to a minimum reported to occur between 200 and 300° C 
(390 and 570° F), which is higher than the temperature of occur- 
rence of minimum elongation and is followed by rapid increase in 
values. Howard (8) also found a tendency for bars broken at 
temperatures between 205 and 315° C (400 and 600° F) to fractiu-e 
in an oblique shearing direction. 



French] 



Steel at High Temperatures 
2. MATERIALS AND METHODS USED 



685 



(a) SteEIyS Tested. — ^The steels tested in this investigation 
were received as half-inch boiler plates of fire box and marine 
grades. The specified tensile strength and composition for each 
are shown in Table i, but the third class listed showed slightly 
higher tensile values than the limits prescribed and was supple- 
mented by class 4. The former was used, how- 
ever, and the tensile properties at high tempera- 
tures of the hot-rolled steel were determined be- 
cause the number of plates of the first two series 
was insufficient for completion of the desired 
tests. 

The steels were made in the basic open hearth 
and the baths kept in a boiHng condition up to 
the moment of casting. Such metal is often re- 
ferred to as " open steel ' ' to distinguish it from 
that which has been "killed" in the ordinary 
manner. It is porous in its cast condition and 
shows some segregation, but is nearly free from 
pipe. Variations in composition have, however, 
been kept to a minimum by cutting the patterns 
from which the test specimens were machined 
from steel originally in the central and least seg- 
regated portion of the ingot, but unfortunately Fig. 2 ■—Distribution 
no detailed record of this procedure is available of patterns cut from 
except in the case of series 4, where the patterns !^^^ Tiayjireoxp e 
were distributed as given in Figure 3. Check 
analyses show excellent uniformity and agree closely with the com- 
positions shown in Table i . 

TABLE 1.— Steels Tested 



13 


M 


a 




JO 


// 


IZ 




7 


e 


9 




4 


s 


6 




/ 


z 


3 










) 



Series 


Grade 


Specified 
tensile 
strength 


Composition 


C 


Mn 


P 


S 


1 


A. S.T.M. firebox! 


I-bs./in.2 
52 000-62 000 
60 000-70 000 
45 000-55 000 
45 000-55 000 


Per cent 

0.19 
.25 
.17 
.18 


Per cent 

0.43 

.38 

.36 

.43 


Per cent 

0.020 
.019 
.024 
.017 


Per cent 

0.031 


2 


Marine 


.031 


3 




.031 


4 


Railway fire box. 


.035 









1 American Society for Testing Materials, fire-box steel. Specification A 30-18. 

102314°— 22 2 



686 



Technologic Papers of the Bureau of Standards [Voi. i6 



A detailed description of ingots and plates produced in the 
manner referred to above is contained in a report by Charles 
Huston/ which includes many excellent. photographs and charts 
showing the porosity of the cast metal and the chemical and 
physical characteristics of the rolled plates. 

TABLE 2.— Ingot Size and Rolling Record of Steels Tested 



Series 


Original 
weight 


Ingot 
size 


Reduc- 
tion in 
rolling 


Pattern 
size 


1 


Pounds 
5900 
5650 
3150 
3150 


Inches 
36 by 15 
32 by 12 
26 by 12 
26 by 12 


30tol 
24tol 
24tol 
24tol 


Inches 
36 by 18 


2 


36 by 12 


3 


36 by 18 


4 


40 by 15 







In Table 2 is given a record of original ingot size and reduction 
in rolling the various plates tested in this investigation. Flat test 
bars, with long dimension in the direction of rolling, were cut from 
patterns taken from these plates and machined to the form shown 
in Figure 4. 



r — i V s 



j>'i^ 



r-M 



u 



Over 



2-M' 






+-;4- 



■Thermoeoi/ple Hole 



T 
_L 



-z'k- 



-^54- 






-Onr . 



Fig. 4. — Form, and dimensions of test specimen used 

(b) Apparatus for Determination of ProportionaIv I^imit. — 
At the outset emphasis is laid on the fact that, for the work re- 
quired throughout the various sections of this investigation, suita- 
ble and readily manipulated apparatus and not the most accurate 
mechanism available was sought. The material under test is 
lacking in entire uniformity (which condition is usual in engineer- 
ing material), so it appeared undesirable to construct elaborate 
equipment requiring a great deal more time in development and 
actual test. 



5 C. L. Huston, Experiments on the segregation of steel ingots in its relation to plate specifications, 
Proc. Am. Soc Test. Mat. 6 (1906), p. i8a. 



Technologic Papers of the Bureau of Standards, Vol. 16. 




Fig. 5. — Apparatus used for determining proportional limit 




Fig. 8. — Apparatus for determining tensile properties of metals at high temperatures 



French] 



Steel at High Temperatures 



687 




H • Specimen 

B - Yoke 

Fig. 6. — Yoke 



The apparatus used in determination of the limit of proportion- 
ality at various temperatures is shown in Figure 5 and consisted 

primarily of two aluminum-alloy frames each 

rigidly fastened to a quenched and tempered steel 

yoke (shown in Fig. 6) by two annealed low-carbon 

steel rods. The specimen passed freely through 

the holes in the base of each of the frames. Yokes 

were each clamped to the specimen by three 

quenched and tempered high-speed steel screws, 

while the spreading of the former was overcome 

by the long screw. The flanges on the upper frame 

were so arranged that dial micrometers for indicating deformation 

might readily be securely fastened to them, while those of the 

lower frame were capped with 
polished steel plates to give a 
smooth bearing surface to the 
plungers of the dials. 

The smallest division on the 
instruments used was equal to 
o.ooi inch, but estimated read- 
ings to the nearest o.oooi inch 
were readily obtained. When 
stress is appUed to the speci- 
men, half the algebraic sum of 
the deformation recorded by 
the two dials represents the 
deformation of the specimen, 
which is centrally located with 
respect to the entire apparatus. 
For example, upon application 
of load the apparatus may twist 
to some extent, the dial on the 
left showing a negative defor- 
mation (decrease in length) of 
0.004 inch, while that on the 
right registers a positive defor- 
mation (increase in length) of 
0.009 inch. Half the algebraic 
sum (14 ( + 0.009 inch -0.004 

inch) =0.0025 inch) represents the deformation (increase in length) 

of the specimen under the load appHed. 




Inner Tube OAd 
Base (wrided) 
Outer Tube 
Top Plate 

Micahite and 
Ibbestos Ihsulatkit 

Infusorial £artl> 

F Terminals 

6 Niareaie fieslatori 



Fig. 7. — Heating furnace 



688 Technologic Papers of the Bureau of Standards [Voi. i6 

(c) Heating Furnace. — ^The test specimens were heated by 
means of an electric tube furnace of the form shown in Figure 7. 
Two spiral resistors in series were used. The one covered the 
entire length of the inner tube (11 inches) and the other was concen- 
trated at the ends, the two requiring about 80 feet (24 m) of 22- 
gage nichrome wire. Yokes and the greater part of the 18-inch 
(46 cm) test bar and rods were contained in the heating chamber, 
which was 11 inches long. A comparatively small temperature 
gradient was obtained under suitable operating conditions, as the 
effective heating length during test was about one-third of this at 
the center of the tube length, or approximately 3 inches. The 
furnace was operated on either no or 220 volts, direct current, 
close regulation being obtained by variable resistance in series in 
the circuit. 

{d) Test Procedure. — ^The method of setting up the apparatus, 
together with procediore followed in actually carrying out the tests, 
was substantially as follows: A specimen was marked on one sur- 
face with a double-pointed center punch leaving marks 2 inches 
apart. Next, the yokes were attached to the specimen by setting 
the single screw into these impressions. Then, by lightly tapping 
the opposite side of the yoke containing the two screws, a Hght 
impression of their exact location on the test bar was obtained. 
These points were then enlarged by the use of the double-pointed 
center punch, and the yokes carrying rods and frames were firmly 
attached to the test piece. 

Bolts holding the upper frame to the two rods were next taken 
off and the upper frame removed. The specimen was then 
passed up through the furnace until the rods appeared above the 
top, when the upper frame was again fastened to the rods. After 
the furnace was placed on a stand and the specimen was in the 
jaws of the testing machine the dials were attached to the frame 
and adjusted to zero. The completely assembled apparatus is 
shown in Figure 8. 

When thermal equilibrium at the desired temperature was 
reached, an initial load of about 1500 lbs. /in. ^ was applied and the 
dials read or, as a matter of convenience, again set at zero. Read- 
ings were then taken at increments of 500 or 1000 poiuids actual 
load until the proportional limit was passed. The dials were 
then removed and the specimen was broken in the usual manner 
with a low rate of extension which approximated the intermittent 
increases of stress applied during determination of the limit of 



French] 



Steel at High Temperatures 



689 



fioem TSmperahirs 



-20 



-IS 



2JJ0O 



/^■^3 

Z9S°C&63'F) 



I5SO0 



A-19 



proportionality. Tests at each temperature were made in dupli- 
cate or triplicate, and the proportional limit was obtained from a 
stress-strain diagram. Typical curves obtained from tests at 
various temperatures throughout the range covered are shown in 
Figure 9. Temperatiure was measured by a 2 2 -gage standardized 
chromel-alumel couple connected to a I^eeds & Northrup portable 
potentiometer. The end of the couple was inserted directly into 
a small hole drilled in the specimen at the fillet, its exact location 
being shown in Figure 4. 

(e) ThbrmaIv Equiubrium. — In order to obtain reliable and 
satisfactory results with the method described in the preceding 
paragraphs, thermal equilibrium must be reached prior to the start 
of the loading and 
maintained during 
the actual 8 to 15 
minutes during 
which the test is be- 
ing carried out. The 
adjustable resistance 
in series in the elec- 
trical circuit makes 
cmrent adjustment 
possible, so that the 
loss of heat from the 
heating tuiit, ends of 
test specimen, and 
auxiliary apparatus 
by radiation, convec- 
tion, and conduction 
balances the energy 
added to this entire system. The effect of temperature variations 
may be large unless care is taken to allow sufficient time for the 
specimen to become uniformly heated throughout after the poten- 
tiometer has once indicated the desired temperature. The dial 
readings will assist in determining when equilibrium has been 
reached and is being maintained. 

Temperature determinations under actual test conditions, made 
by placing thermocouples in holes located at various points in a 
specimen carrying entire auxiliary apparatus, show that the posi- 
tion chosen for the single thermocouple (in the fillet) is representa- 
tive of about the mean of the gradient throughout the gage length, 






I0 70O 



-S 



.001 .001 

IM/t Sfformatioit - I/idtes per JncA 

P^G. g. — Typical stress-strain diagrams obtained at various 
temperatures 



690 



Technologic Papers of the Bureau of Standards Woi. 16 



where the teraperature gradually decreased from top to bottom 
(see Fig. 10 for partial reproduction of these variations). This 
variation is within 30° C (54° F) . It is the greatest in the upper 
temperature ranges under consideration, and does not exceed 20° C 



Desired temperature, 
degrees Centigrade 



165. 
320, 
400. 



Temperature of specimen at 


Time 


Average 
temp era- 
ture of 
couples 
2, 3, 4, 
and 5, 
degrees 
Centi- 
grade 


positions indicated, degrees 


after 


Centigrade 






couple 
No. 1 
first 
reached 
desired 
tempera- 
ture, 
minutes 


1 


2 


3 


4 


5 


165 


173 


167 


158 


165 


15 


166 


320 


327 


327 


310 


322 





322 


325 


334 


334 


320 


329 


5 


329 


325 


336 


334 


318 


332 


20 


330 


402 


415 


412 


393 


402 


10 


405 


402 


415 


412 


393 


402 


20 


405 



Maxi- 
mum 
tempera- 
ture va- 
riation, 
degrees 
Centi- 
grade 



IS 

17 
14 
16 

22 
22 



T; 



Fig. 10. — Temperatures at variotis parts of test specimen 
Jlegrees C 




Segreef F 

Fig. II. — Tensile properties of half-inch A. S. T. M. firebox boiler plate at elevated 

temperatures {Series i) 

Plates rated as 32-62 000 potmds tensile strength. Tested as rolled. Curves are based on averages of 
several tests at each temperature chosen. Carbon, 0.19; manganese, 0.43; phosphorus, 0.020; and sulphur, 
0.031 per cent. 

(36° F) at the lower temperatures used. However, as the ther- 
mocouple, specimen with auxiliary apparatus, and fiumace are in 
the same relative position in each test, the results obtained at 
various temperatures throughout the range 20 to 465° C (70 to 
870° F) are comparable. 



French] 



Steel at High Temperatures 



691 



3. TENSILE PROPERTIES OF HOT-ROLLED BOILER PLATE AT ELEVATED 

TEMPERATURES 

As a basis for comparison with steels subjected to deformation 
in various ways tensile tests were made on the four grades listed 
in Table i . Results obtained are graphically represented in Figures 
II, 12, 13, and 14. In all grades of plates increase in temperature 
above the ordinary atmospheric range is accompanied by distinct 
changes in strength and ductility, namely: 

(a) Tensile strength decreases a few thousand pounds per square 
inch in the neighborhood of 95° C (200° F). This is followed by 
an increase to a maximum, which occurs at 290° C (550° F) in 



negrees C 




Uegrets F 



Fig. 12. — Tensile properties of half -inch marine boiler plate at elevated temperatures 

(Series 2) 

Plates rated at 60-70 000 pounds tensile strength. Tested as rolled. Curves are based on averages of 
several tests at each temperature chosen. Carbon, 0.25; manganese, 0.38; phosphorus, 0.019; and sulphur, 
0.031 per cent. 

plates of the first three series and at about 250° C (480° F) in 
series 4, representing plates of lowest tensile strength. With 
further increase in temperature the strength decreases, and again 
approximates ordinary atmospheric temperature values in the 
range 370 to 400° C (700 to 750° F). 

(6) The limit of proportionality increases and is a maximum in 
the neighborhood of 150° C (300° F). In the case of the fire-box 
grade plates this increase is more marked, and is maintained above 
room temperature value to a higher temperature than is the case 
with the marine plate, which has, in effect, constant proportional 
limit up to about 175° C (350° F). While such differences are 
noticeable at these relatively low temperatures, the proportional 
limit of the higher tensile strength marine plate is practically the 



692 



Technologic Papers of the Bureau of Standards {Vci.16 



same at 465° C (870° F) as that of series i and 3 fire-box grade 
plates and but slightly higher than that of the fourth series (rail- 
way fire-box plate of lowest tensile strength) . 




Uegrees F 

Fig. 13. — Tensile properties of half-inch railway firebox boiler plate at elevated temr 

peratures {Series j) 

Plates rated as 45-55 000 pounds tensile strength. Tested as rolled. Curves are based on averages of 
several tests at each temperature chosen. Carbon, 0.17; manganese, 0.36; phosphorus, 0.024; and sulphur, 
0.03 1 per cent. 

(c) Only a slight decrease in elongation is observed until a 
temperature of about 95° C (200° F) is reached, above which 




Hegrees F 

Fig. 14. — Tensile properties of half -inch railway firebox boiler plate at elevated tem- 
perature {Series 4) 

Plates rated as 45-55 000 pounds tensile strength. Tested as rolled. Curves are based on averages of 
several tests at each temperature chosen. Carbon, 0.18; manganese, 0.43 ; phosphorus, 0.017; and sulphur, 
0.035 per cent. 

the rate of decrease is much higher and a minimum is reached 
at about 245° C (470° F). Elongation then increases but does 
not throughout the range under consideration reach the ordinary 



Frenci] Steel at High Temperatures 693 

atmospheric temperature value in the marine plate (rated 60 000- 
70 000 pounds tensile strength) and but slightly exceeds its room 
temperature value in the case of the fourth series at 465° C 
(870° F). 

(d) Reduction of area closely follows the inflections registered 
in the curves for elongation but reaches a minimum at a slightly 
higher temperature, except in the case of the lowest tensile 
strength fire-box plate where the minimum occurs at practically 
the same temperature as that for elongation. At 465° C 
(870° F) reduction of area is greater than the value obtained at 
atmospheric temperature in each of the four series of plates 
tested. 

It is to be noted that maximum tensile strength does not coin- 
cide with minimum reduction of area or maximum proportional 
limit, but examination of Figures 11 to 14, inclusive, indicates 
that the inflections in the curves for reduction of area are, in 
general, more nearly coincident with the reverse inflections in 
the curves for tensile strength, and that elongation and propor- 
tional limit may be similarly paired. 

III. EFFECT OF PERMANENT COLD AND BLUE DEFORMA- 
TION ON THE TENSILE PROPERTIES OF STEEL AT VARI- 
OUS TEMPERATURES 

1. COLD-ROLLING 

The effect of cold work on the tensile properties of steel at 
ordinary temperatures is to increase the elastic properties and 
to a smaller degree the tensile strength with an accompanying 
decrease in ductility as measured by elongation and reduction 
of area. The greater the total reduction within the capacity 
of the material the greater is the increase in strength. The 
tensile properties of steel at elevated temperatures are likewise 
modified by such cold deformation. Jeffries (33) reports in- 
creased strength at blue heat (200 to 300° C) when Armco iron 
is drawn at room temperature with moderate reductions, but 
with 96 per cent reduction of area by cold-drawing the tensile 
strength is greater at room temperature than at any higher one. 

The tensile properties at various temperatures of cold-rolled 
fire-box and marine grades of boiler plate are shown in Figures 15 
and 16. Comparison between the cold and hot rolled properties 
is also shown in Figure 17. 
102314°— 22 3 



694 



Technologic Papers of the Bureau of Standards \Voi.z6 




too 3eo 

Degrees C 



«» 



Fig. 15. — Tensile properties of cold-rolled A. S. T. M. firebox boiler plate at various 

temperatures (Series I steel) 

Carbon, 0.19; manganese, 0.43; phosphorus, 0.020, and sulphur, 0.031 per cent. Plates reduced cold 
■^ inch from Ji inch thickness 




Fig. 16. — Tensile properties of cold rolled marine boiler plate at various temperatures 

{Series 2 steel) 

Carbon, 0.23; manganese, 0.38; phosphorus, 0.019; and sulphur, 0.031 per cent. Plates reduced cold, 
■^ inch from J4 inch thickness. 



French] 



Steel at High Temperatures 



695 



One-sixteenth inch "cold reduction," approximating 12.5 per 
cent of the original plate thickness, increases the tensile strength 
at room temperature about 20 per cent. It also increases the 
strength of the hot-rolled plates up to about 465° C (870° F) by a 
similar amount, showing that this effect is maintained until rela- 
tively high temperatures are reached. 

The changes in proportional limit are more marked and of con- 
siderable interest. At ordinary temperatures an increase of about 




\ I ti I, I I jl I 
joa ^w j» w M> 



/it 



too 

300 



700 \9oo ma 'P 
■MO *e 



tlao 
Temperature 

Fig. 17. — Comparison of tensile properties at various temperatures of cold and hot' 
rolled firebox and marine boiler plate 

80 to 95 per cent is shown. Similarly, an increase of 60 to 100 
per cent above the values obtained in tests of hot-rolled plates is 
found at temperatures up to and including 245° C (470° F). In 
the blue-heat range, 295° C (565° F), the increase in proportional 
limit due to cold work reaches the very high value of 150 per cent 
jn the marine plate and nearly 200 per cent in the fire-box grade. 
This, however, is not accomplished at the expense of ductility, as 
the relation between elongation of cold and hot rolled plates has 
also increased. The relation between reduction of aiea of cold 



696 



Technologic Papers of the Bureau of Standards \voi. 16 



and hot finished marine steel has likewise increased to some extent, 
while that for the fire-box grade has decreased but slightly. 

Bregowsky and Spring (25) in a report of tests of cold-rolled 
Bessemer shafting show secondary inflections in their tensile prop- 
erties — temperature curves similar to those described above 
though occurring at somewhat lower temperatures. However, no 
direct comparison with the hot-rolled steel is available. 

In order to determine whether these changes at blue heat are 
maintained at ordinary temperatures specimens were annealed at 
successively increasing temperattires and then tested in the usual 



so 



79 



|J9 




5 



I 



ibOTW 



■^ 



eo(f 



A. 



lot 200 300 

Degree C 
tlwealms Temperati/te 



^900 



20 



Fig. i8.- 



-Effect of partial annealing on the tensile properties of cold-rolled railway 
firebox boiler plate {Series 4 steel) 

Carbon, 0.17; manganese, 0.36; phosphorus, 0.024; and sulphur, 0.031 per cent. Plates reduced cold, 
■^ inch from i4 inch thickness. Held 30 minutes at annealing temperature and air cooled. 

manner at the temperature of the room. The results, which are 
graphically represented in Figure i8, illustrate, as far as the tensile 
properties are concerned, the benefits derived from the "bluing" 
of cold-finished products, such as thin- wall seamless steel tubes, 
and show the effects of final partial annealing on low-carbon steel 
cold rolled in the ordinary manner. Short- time annealing in the 
blue-heat range, 295° C (565° F), has little effect on the tensile 
strength and elongation but materially increases the elastic ratio 
with only a minor decrease in reduction of area. 

In the case of the fire-box steel under test this increase in the 
limit of proportionality is about 20 per cent. It is likewise 



French] Steel at High Temperatures 697 

apparent from Figure 18 that a short time annealing at somewhat 
higher temperatm-e, 405° C (765° F), accomplishes little, if any- 
thing, in the improvement of the tensile properties of this cold- 
rolled steel. 

2, BLUE ROLLmG 

Working steel in the ranges where ordinarily temper colors are 
obtained has long been considered deleterious and even dangerous. 
Stromeyer (37) warns against such practice and, in referring to 
his series of tensile and bending tests on iron, mild and hard steels, 
writes : 

All these results point unmistakably to the great danger which is incurred if iron 
or steel is worked at a blue heat. * * * It is very common practice amongst 
boiler makers to "take the chill out of the plate" if it requires a little setting, or to 
set a flanged plate before it is cold. This is really nothing else than working it at a 
blue heat and should not be allowed. * * * All hammering or bending of iron 
and steel should be avoided, unless the metals are either cold or red-hot. Where 
this is impossible and where the plate or bar has not broken while blue-hot, it should 
be subsequently annealed. 

Howe (38) in 1891 summarized available information relating to 
what he calls "blue shortness" and states in part: 

Not only are wrought iron and steel much more brittle at a blue heat' than in the 
cold or at redness, but, while they are probably not seriously affected by simple ex- 
posure to blueness, even if prolonged, yet if they be worked in this range of tempera- 
ture they remain extremely brittle after cooling and may indeed be more brittle then 
than while at blueness. This last point, however, is not certain. 

The loss of ductility as measured by endurance of bending and drifting is enormous. 
That this is not due to incipient cracks is shown by the simultaneous increase of 
tensile strength and by the restoration of ductility by armealing. The effect of blue 
working on ductility as measured by elongation (on rupture by static tensile stress) is 
ver>' irregular and apparently anomalous. * * * Heating to redness may com- 
pletely remove the effects of blue working. 

Howe further called attention to the resemblance between the 
effects of cold working and those of blue working, and that the 
immediate effect of these two operations might be suspected to 
be identical in natiire. He states: 

It is true that the gain in elastic limit does not seem to excel that in tensile strength 
as markedly in the case of blue as in the case of cold working, nor is it clear that the 
tensile strength and elastic limit increase during rest after blue as they do after cold 
working. But this is natural, for we see reason to believe that heating cold-worked 
iron to blueness greatly accelerated the changes which cold working starts, so that, 
when this change is started by distortion at blueness instead of in the cold it may 
occur so rapidly and so nearly reach its full growth before the metal grows cold that 
no considerable further change occurs thereafter. The effects of blue working are 
more intense and more injurious than those of cold working. 

' The terms " blue work," " blue shortness," etc., as used by Howe, refer to temperatures where ordina- 
rily temper colors are obtained and may be considered to be within the range 220-320° C (430-600° F). 



698 



Technologic Papers of the Bureau of Standards [Vou 16 



Ridsdale (40) referred to blue heat, 315-370° C (600-700° F), 
as the state of minimum plasticity and showed that soft steel 
developed brittleness when worked or soaked in a furnace for a 
long time in this range. He further pointed out that by reheating 
to a suitable temperature the good qualities of such steel were 
restored. 

Kurzwemhart (39) reported that a brittle boiler plate returned 
to the Teplitz Steel Works gave rise to investigation showing that 
the plate had been removed from the furnace at an tmeven tem- 
perttu"e extending from red-hot on one side to brown-hot on the 
other. Experiments showed that not only was the blue heat 
dangerous, but working the metal at certain other temperatures 
was likewise deleterious. Bending tests showed the most dan- 
gerous temperature to be that at which the surface assumed a 
light yellowish coloration. Blue brittleness could be removed, 
however, by heating to a dull red such as could only be observed 
in a darkened room. 

Jeffries (34) has studied the effects of blue work on the tensile 
properties of Armco iron and reached the following conclusions : 

1. Armco iron deformed at room temperature a given amount does not increase 
as much in tensile strength as when deformed the same amount at blue heat. 

2. The effect of drawing Armco iron at 200 to 400° C is to produce greater tensile 
strength at all temperatures up to 550° C than would obtain with the same amount 
of deformation in the cold. The elongation is less after drawing at 200 to 400° C 
than after drawing at room temperature. The same conclusion is true, in general, of 
the reduction of area. 

Jeffries also reports results obtained in private communication 
from W. E- Ruder to show the effects of rolling cold and at 
various elevated temperatures on the strengths of mild steel 
and annealed nickel-chromium steel. These data are reproduced, 
respectively, in Tables 3 and 4. 

TABLE 3. — Tensile Strength of Mild Steel Drawn at Various Temperatures * 



Size and condition of material 


Tensile 

strength 


Size and condition of material 


Tensile 
strength 


0.192 inch diameter 


Lbs./in.2 
58 200 
55 200 
67 000 
69 500 
110 200 
112 300 


0.192 inch diameter 


Lbs./in.» 
112 700 




Reduced to 0.179 inch at 300° C 


113 500 


n.lQ^ iiirVi diampfpr. . 


0.192 inch diameter 


114 100 


Reduced to 0.179 inch cold 


Reduced to 0.179 inch at 400° C 


105 300 


0.192 inch diameter 






Reduced to 0.179 inch at 240° C 









' Zay Jeffries, Physical changes in iron and steel below the thermal critical range, Mining and Metallurgy, 
158 (1920), section No. 20. 



French'^ 



Steel at High Temperatures 



699 



TABLE 4. — Tensile Properties of a Nickel-Chromium Steel Heat-Treated and 

Worked in Different Ways * 



Condition of material 



Tensile 


Yield 


strength 


point 


Lbs./in.2 


Lbs./in.2 


101 500 


67 100 


137 600 


132 500 


199 100 


163 700 


114 200 


102 000 



Percent- 
age elon- 
gation 
in 2 
inches 



Annealed 

Annealed, then reduced 10 per cent at 300° C . 
Heat treated and reduced 10 per cent at 300° C 
Annealed and reduced 10 per cent cold 



25.5 

10.5 

7.5 

19.5 



' Zay Jeffries, Physical changes in iron and steel below the thermal critical range, Mining and Metallurgy, 
158 (1920), section No. 20. 

While there seems to be unanimity of opinion that blue work 
is deleterious to steel, Howe (38) early called attention to the 
fact that — 

Millions of car axles, blue from " hot boxes, ' ' are chilled with snow and jarred under 
heavy load at loose rail joints, yet are apparently unharmed. Among the many 
thousand steel boilers tens of thousands of plates must have been worked more or 
less at blueness, yet failures are rare * * *. Again, though many recognize 
that machine riveting has a great advantage over hand riveting, in that its work ceases 
before the rivet cools to blueness, while the hand riveter usually continues hammer- 
ing while the rivet is passing blueness, yet relatively few hand worked rivets fail in 
use * * *. Finally, much crucible steel in the form of bars, plates, etc.,' is 
habitually rolled or hammered till its temperature has fallen below visible redness. 

More recently the question of effects of deformation at tempera- 
tures below the thermal transformations has become of interest 
in connection with the straightening of crank shafts for airplane 
engines while cooling from the tempering heat. Whether such 
work is detrimental or can safely be appUed to these and other 
types of forgings has for some little time been under discussion. 

In order to determine the effect of permanent deformation at 
blue heat on the high-temperature properties of boiler plate 
several patterns were reduced at about 300° C from ^ inch to \^ 
and ^ inch, respresenting, respectively, 6.25 and 12.5 per cent 
reduction in thickness. The patterns were rolled in a single- 
stand 16-inch mill, a number of light reductions to effect the 
totals mentioned above being used. Considerable difficulty was 
encountered at first in producing flat plates suitable for test; 
probably, in the main, because of the short length of the patterns. 
An attempt was made to straighten the first plates cold and also 
at blue heat immediately after rolling, but they broke transversely 
with a very coarsely crystalline fracttire as soon as pressure was 
applied. 



700 



Technologic Papers of the Bureau of Standards [Voi. i6 



It was found after discarding these plates that all the patterns 
of series i and 2 steels had been used, so that it was necessary to 
carry out the desired tests on series 4, railway fire-box steel, 
quite similar in composition and properties to series 3. This, 
however, does not affect consideration of the relative effects of 
blue and cold work as shown in Table 5 based on data represented 
graphically in Figures 17 and 19. 

TABLE 5.— Effect of Blue and Cold Rolling on the Tensile Properties of Fire Box 
Boiler Plate at Various Temperatures 





Temperature of tests 






























Hot- 
rolled 

(series 1) 


1/16-inch 




Hot- 
rolled 
(series 4) 


1/32-inch 




1/16-inch 




Hot 


Cold 


Blue 






"cold 
reduc- 


Ratio 1 


"blue 
reduc- 


Ratio 1 


"blue 
reduc- 


Ratio I 


rolled 


rolled 


rolled 


Average 


tion" 




tion" 




tion" 




steel 


steel 


steel 
























»c 


°C 


°c 


»F 


Tensile strength, pounds per square inch 


°C 


















21 


21 


21 


21 


70 


59 000 


71130 


1.21 


55 600 


72 000 


1.29 


79 800 


1.43 


91 


89 


91 


90 


194 


55 530 


69 300 


1.25 


52 800 


68 900 


1.30 


77 700 


1.47 


156 


156 


156 


156 


313 


58100 


70100 


1.21 


63 800 


69 400 


1.09 


78 300 


1.23 


243 


243 


243 


243 


469 


65130 


72 930 


1.12 


65 800 


74 300 


1.13 


79 100 


1.20 


295 


295 


295 


295 


563 


66 700 


75 540 


1.13 


65 200 


73 650 


1.13 


78 000 


1.20 


407 


402 


407 


405 


761 


49150 


61850 


1.26 


47 800 


53 850 


1.12 


57 700 


1.21 


465 


463 
21 


463 
21 


463 
21 


865 
70 


41850 


50 400 


1.20 


39 900 


43 600 


1.09 


44 400 


1.11 




Proportional limit, pounds per square inch 


21 


23 300 


45 130 


1.94 


14 500 


42 800 


2.95 


53 500 


3.69 


91 


89 


91 


90 


194 


23 630 


48 550 


2.05 


21200 


52 500 


2.48 


60 200 


2.84 


156 


156 


156 


156 


313 


26 600 


47 450 


1.78 


25 100 


48 350 


1.93 


58 800 


2.34 


243 


243 


243 


243 


469 


24 900 


41700 


1.67 


18 500 


48 500 


2.62 


57 200 


3.09 


295 


295 


295 


295 


563 


15 250 


44 630 


2.92 


15 000 


34 200 


2.28 


54 800 


3.65 


407 


402 


407 


405 


761 


12 960 


32 000 


2.66 


11500 


29 650 


2.58 


40 200 


3.50 


465 


463 
21 


463 
21 


463 
21 


865 
70 


11430 


26 050 


2.36 


9 400 


22 750 


2.42 


27 400 


2.92 




Percentage elongation in 2 inches 


21 


37.8 


18.2 


0.48 


41.0 


22.2 


0.54 


18.0 


0.44 


91 


89 


91 


90 


194 


34.7 


16.4 


.47 


37.5 


17.5 


.47 


15.1 


.40 


156 


156 


156 


156 


313 


24.9 


12.8 


.52 


29.6 


17.3 


.58 


13.3 


.45 


243 


243 


243 


243 


469 


19.9 


13.4 


.68 


25.8 


18.2 


.70 


15.1 


.58 


295 


295 


295 


295 


563 


25.7 


17.5 


.68 


31.0 


25.0 


.81 


16.4 


.53 


407 


402 


407 


405 


761 


33.8 


23.6 


.70 


43.0 


32.8 


.76 


. 25.8 


.60 


465 


463 
21 


463 

21 


463 
21 


865 
70 


39.2 


23.8 


.61 


43.5 


34.2 


.79 


27.9 


.64 




Percentage reduction of area 


21 


57.1 


46.7 


0.82 


68.2 


50.7 


0.74 


46.2 


0.68 


91 


89 


91 


90 


194 


58.3 


43.2 


.74 


63.4 


47.7 


.75 


43.4 


.68 


156 


156 


156 


156 


313 


49.3 


38.6 


.78 


54.3 


41.0 


.75 


37.3 


.69 


243 


243 


243 


243 


469 


45.1 


37.3 


.84 


54.7 


41.2 


.75 


36.3 


.66 


295 


295 


295 


295 


563 


45.6 


36.5 


.78 


56.8 


46.6 


.82 


42.6 


.75 


407 


402 


407 


405 


761 


60.7 


51.2 


.84 


73.0 


62.4 


.85 


57.5 


.79 


465 


463 


463 


463 


865 


67.7 


57.2 


.84 


76.8 


66.0 


.86 


63.6 


.83 



1 In each case the ratio given is the blue or cold rolled properties to the hot-rolled properties as determined 
by tests on patterns of the same plate. 

I . The increase in strength at room temperature resulting from 
"cold reduction" of 12.5 per cent is about the same as that 
produced by half this reduction (6.25 per cent) at blue heat. 



French] 



Steel at High Temperatures 



701 



This, in general, is also true at temperatures up to 295° C (565° F) 
(blue heat) . Above this temperattue the increase in strength 
resulting from "blue deformation" is somewhat less than that 
from cold rolling. 

2. A "blue reduction" of twice the amount given above (6.25 
per cent) does not increase the strength at room or elevated 
temperatures proportionally. 




Fig. 19. — Tensile properties of blue-rolled railway firebox boiler plate at various tempera- 
tures {Series 4 steel) 

Carbon, 0.18; manganese, 0.43; phosphorus, 0.017; ^nd sulphur, 0.035 P^r cent. Plates reduced at about 
300° C from 14 inch thickness. 

3. At 245° C (470° F) the strength of the blue worked steel is 
little in excess of that hot rolled and about the same whether the 
"blue reduction" is 6.25 or 12.5 per cent. 

4. The maximum tensile strength of the steel subjected to 
6.25 per cent "blue reduction" is obtained at blue heat, 295° C 
(565° F), and the shape of the tensile, strength temperature 
curve shown in Figure 19 is similar to that for the cold-rolled 

102314°— 22 4 



702 Technologic Papers of the Bureau of Standards [Voi. ns 

steel, while with twice this "blue reduction" (12.5 per cent) 
about the same strength is obtained at 295° C (565° F) as at room 
temperature. In both cases, however, the tensile strength tem- 
perature curves change direction quite abruptly at or about 295° C 

(565°F)._ 

5. The increase in the limit of proportionality at room temper- 
atiure resulting from 6.25 per cent "blue reduction" is greater 
than that obtained from twice as much "cold reduction." This 
is also true up to 245° C (470° F), above which temperature the 
increase resulting from cold work is about equal to that produced 
by the blue work in question. 

6. The proportional limit increases with first rise in temperature 
and is a maximum at about 90° C (195° F), after which it decreases. 
The form of the proportional limit temperature curve for steel 
reduced 6.25 per cent at blue heat is similar to that reduced twice 
this amount in the cold, but the secondary increase which in the 
cold worked steel appears to attain its maximum at about 340° C 
(645° F) occurs at 245° C (470° F) in the blue worked metal. 
This inflection is not observed in the steel subjected to 12.5 per 
cent "blue reduction," the proportional limit decreasing slowly 
from 90 to about 245° C (195 to 470° F) and more rapidly there- 
after. 

7. Elongation and reduction of area decrease to a minimum in 
the neighborhood of 200° C (410° F) and thereafter increase to 
high values at 465° C (870° F) greatly in excess of those obtained 
at room temperature. There appears to be a more rapid increase 
in elongation between 295 and 410° C (565 and 770° F) coincident 
with the rapid decrease of proportional limit mentioned above for 
steel reduced 6.25 per cent at blue heat. 

The effect of partial annealing of blue worked steel is shown in 
Figure 20. As the temperature increases the strength at room 
temperature decreases, and this is accompanied by increase in 
elongation and reduction of area. The proportional Umit de- 
creases until an annealing temperature of about 500° C (930° F) 
is reached, but with slow cooling from above this to about 600° C 
(1110° F) the elastic ratio is greatly increased. Annealing for 
30 minutes at about 730° C (1345° F) completely removes the 
effects of ' ' blue deformation. ' ' 



French] 



Steel at High Temperatures 



703 



3. DEPTH OF PENETRATION OF EFFECT OF BLUE AND COLD ROLLING. 

Since the effects of blue and cold work are so marked and 
maintained over a considerable temperature range, the question 
naturally arises as to whether the increased strength of the plate 
is due to hardening of the "skin," so that there is a decrease in 
strength from surface to center or whether the magnitude of the 
observed effect is substantially the same throughout the cross 
section, especially in light plates such as are under investigation. 
The surfaces of cold and blue rolled bars were accordingly milled 
to progressively increasing depths and tested in the usual manner 
at room temperatiure. 




Hmea/ing Timperatgre 

Fig. 20. — Effect of partial annealing on the tensile properties of blue-rolled railway fire- 
box boiler plate (Series j steel) 

Carbon, 0.17; manganese, 0.36; phosphorus, 0.024; and sulphur, 0.031 per cent. Reduced 1/32 inch at 
about 300° C from 14 inch thickness. Held 30 minutes at annealing temperatures and air cooled. 

' The results of these tests are given in Table 6 and show that 
there is substantially no difference in the strength factors when 
even a considerable depth of surface metal has been removed, 
whereas the ductility as measured by elongation and reduction of 
area gradually decreases with removal of increasing layers of metal. 
The origin of this effect is, without doubt, due to the decrease 



704 



Technologic Papers of the Bureau of Standards [Voi. 16 



in thickness of the specimen, while the width has been kept con- 
stant.^ 

TABLE 6.— Tensile Properties of Cold and Blue Rolled Fire-Box Boiler Plate upon 
Removing Successively Increasing Depths of Surface Metal (Series 3 steel) 

REDUCED 1/16-INCH COLD FROM 1/2-INCH THICKNESS 



Sample number 


Sample 
thick- 
ness 


Surface 

metal 

removed 


Propor- 
tional 
limit 


Tensile 
strength 


Percent- 
age elon- 
gation in 
2 inches 


Percent- 
age re- 
duction 
of area 


E13 


Inch 
0.437 
.437 


Inch 


Lbs./in.2 
52 000 
47 200 


Lbs./in.' 

82 200 
74 800 


18.5 
18.5 


47.4 


E20 




47.1 










.437 




49 600 


78 500 


18.5 


47.2 








E14 


.377 
.371 




54 500 
56 500 


74 900 
78 600 


18.0 

17.0 


46.9 


E19.. 




41.2 










.374 


0.031 


55 500 


76 750 


17.5 


44.0 






E18 


.317 
.310 




54 000 
53 000 


76 400 
76 000 


15.5 
17.0 


44.3 


E15 




41.5 








Average ,. _, 


.313 


.062 


53 500 


76 200 


16.2 


42.9 






E2S 


» .250 
.252 




56 000 
59 000 


77 300 
76 500 


14.0 
13.0 


39.8 


Ell 




40.0 








Average 


.251 


.093 


57 500 


76 900 


13.5 


39.9 






E21 


.188 
.192 




53 500 
53 500 


77 900 
77 500 


14.5 
13.0 


36.8 


E16 




34.5 








Average 


.190 


.124 


53 500 


77 700 


13.8 


35.6 






REDUCED l/ie-INCH AT I 


BLUE HE 


AT FRO] 


M 1/2-INC 


:H THICKNESS 





P3 


0.438 
.442 




70 000 
69 500 


94 600 
94 300 


13.0 
12.5 


33 7 


P9 




38.6 








Average 


.440 




69 750 


94 450 


12.8 


36.2 








Pll 


.377 
.375 




70 000 
66 000 


94 700 

95 600 


11.5 
10.5 


38 2 


PIO 




35.3 








Average 


.376 


.032 


68 000 


95 150 


11.0 


36.8 






P8 


' .311 

.311 




68 500 
70 000 


95 700 
95 700 


9.5 
10.5 


36.0 


P4 




36.6 








Average 


.311 


.064 


69 250 


95 700 


10.0 


36.3 






P12 


.249 
.250 


" 


66 000 
73 000 


95 200 
94 800 


9.0 
8.0 


32.0 


P13 




31.2 








Average 


.250 


.095 


69 500 


95 000 


8.5 


31.6 






P2 


.185 
.185 




71000 
70 000 


96 400 
96 400 


6.5 
6.5 


22.4 


PI 




23.0 








Average 


.185 


.128 


70 500 


96 400 


6.5 


22.7 







4. TENSILE TESTS OF TRANSVERSE SPECIMENS OF HOT, COLD, AND 
BLUE ROLLED BOILER PLATE AT VARIOUS TEMPERATURES 

Tests of longitudinal specimens taken from plates rolled at 
various temperatures do not fully define the effects of such work 
even when only considering the tensile properties. Accordingly 



' H. L. Moore, Tension tests of steel vrith test specimens of various size and form. Report of subcom- 
mittee to Committee E i. Proc. Am. Soc. Test. Mat. 1918, part i, p. 403. 



French] 



Steel at High Temperatures 



705 



tests were made on samples taken transversely from hot, cold, and 
blue rolled fire-box steel, series 4, Table i . The results are given 
in Figxire 21 and summarized to show the high temperature 
comparisons in Table 7. 




T<^mperature 

Fig. 21. — Tensile properties of hot-, cold-, and blue-rolled railway firebox boiler plate at 
various temperatures as determined on transverse test specimens {Series 4 steel) 

Carbon, 0.18; manganese, o. 43; phosphorus, o. 017; and sulphur, o. 03s per cent. Cold- and blue-rolled 
plates reduced -^ inch from % inch thickness, respectively, at room temperature and at about 300 C°. 

It is at once evident that the general form of the tensile prop- 
erties temperature curves is similar to those for longitudinal tests, 
with the exception of the limit of proportionality, which does not 
increase with the first rise in temperature. In general, the first 
changes are slight, but there is a decrease in hot and cold rolled 
steel, while in that rolled at blue heat there is a decided tendency 
for the limit of proportionality to remain at nearly its room tem- 
perature value over a considerable range. The agreement between 
duplicate determinations of this factor at slightly elevated tem- 
peratures is not so good as that obtained imder similar conditions 



7o6 



Technologic Papers of the Bureau of Standards [Voi.ja 



in longitudinal samples, so that the values will be considered as 
tentative, and in Fig. 2 1 are shown as dotted lines. 

TABLE 7. — Comparison of Tensile Properties at Elevated Temperatures of Hot, Cold, 
and Blue Rolled Fire-Box Boiler Plates as Determined on Transverse Test Speci- 
mens (Series 4 Steel) 









Temperature 


of test. 








21°C 
(70° F) 


92° C 
(198° F) 


156° C 
(313° F) 


243° C 
(469° F) 


295° C 

(563° F) 


407° C 
(764° F) 


463° C 
(865° F) 


Proportional limit, pounds per square inch: 
Hot rolled 


28 850 

37 000 

47 500 

1.28 

1.65 

56 700 

56 900 

72 750 

1.00 

1.28 

42.8 

36.5 

18.5 

.85 

.43 

62.1 
63.7 
52.2 

1.02 
.84 


27 250 

34 700 

74 000 

1.27 

1.72 

53 300 

55 500 

68150 

1.04 

1.29 

34.8 

26.5 

19.0 

.76 

.55 

61.3 

60.3 

54.9 

.98 

.90 


25 500 

36 900 

44 000 

1.44 

1.72 

62 350 

60 000 

67 250 

.96 

1.08 

23.2 

20.6 

16.2 

.89 

.70 

47.8 
49.2 
46.2 

1.03 
..97 


18 600 

34 700 

42 350 

1.87 

2.28 

67 400 

68 400 
72 300 

1.02 
1.08 

23.3 

21.0 

16.2 

.90 

.69 

46.4 
46.7 
41.0 
1.00 
.88 


16 400 

34 500 

41000 

2.10 

2.50 

66 900 

68 500 

72 250 

1.02 

1.08 

27.3 

19.8 

18.8 

.72 

.69 

46.3 

44.7 

45.6 

.96 

.98 


14 200 

27 500 

40 450 

1.94 

2.74 

50 000 

52 700 

57 700 

1.06 

1.15 

38.5 

31.8 

26.5 

.82 

.69 

67.7 

62.8 

61.3 

.93 

.91 


13 500 


Cold rolled 


21 400 


Bluerolled 


35 000 


Rafio of cold rolled to hot rolled 


1 59 


Ratio of blue rolled to hot rolled 


2.59 


Tensile strength, pounds per square inch: 
Hot rolled 


41 100 


Cold roUed 


42 400 


Blue rolled 


46 500 


Ratio of cold rolled to hot rolled 


1.03 


Ratio of blue rolled to hot rolled 


1.13 


Percentage of elongation in 2 inches: 

Hot rolled 


40.2 


Cold rolled 


33.7 


Blue rolled 


28.2 


Ratio of cold rolled to hot rolled 


.84 


Ratio of blue rolled to hot rolled 


.70 


Percentage reduction of area : 

Hot roUed 


73.6 


Cold rolled 


70.9 


Blue rolled 


67.8 


Ratio of cold rolled to hot rolled 


.96 


Ratio of blue rolled to hot rolled 


.92 







The increase in transverse tensile strength at room temperature 
resulting from blue or cold rolling is very much less than that 
observed in longitudinal tests for the same mechanical reductions. 
However, the relation between the strength of blue, cold, and hot 
rolled steel at room temperature is maintained throughout the 
entire temperature range under consideration. 

Similarly, the increase in proportional limit and decrease in 
ductiHty resulting from deformation at room temperature or 
blue heat are less than the effects observed in longitudinal tests, 
but the ratio of limit of proportionality of cold or blue rolled steel 
to that of the hot rolled metal is greater at blue heat than at room 
temperature. 

The proportional Hmit of steel deformed cold or at blue heat is 
raised more than is the transverse strength, but this difference is 
not so marked as in the case of tests made of longitudinal samples. 

5. PERMANENT DEFORMATION PRODUCED BY STRETCHING 

Huston (42) found that in loading bridge iron to just above the 
"so-called elastic limit" the ductility or toughness remained 
unaffected, so that the metal would yield with every small increase 



French] Steel ct High Temperatures • 707 

in load, while when stressed just below the iron became rigid and 
would not elongate without decided load increase. 

From a large number of tests of wrought iron and low-carbon 
steel Bauschinger (43) made certain empirical deductions regarding 
the behavior of proportional limit and yield point when such 
metals were subjected to overstrain at ordinary temperatures. 
Among these deductions were the following : 

He found that when stretching was produced by a load between 
the proportional limit and yield point the former was raised, 
whereas an applied stress above the yield point resulted in a 
decrease in the Hmit of proportionality. Upon aging for a long 
time at room temperature or for shorter periods at higher tem- 
peratures elevation of this factor was produced. 

Raised proportional limit and yield point brought about by 
aging subsequent to overstrain were again lowered by high heat- 
ing, but the method of cooling was observed to play an important 
part. Rapid cooling was more effective in lowering these factors 
than slow cooling, but the time of rest after heating and cooling 
exerted no further effect. 

Howe (38) reported that stretching at room temperature low- 
ered the limit of proportionality of steel, often to zero, so that if 
retested immediately no proportional limit or a very low one was 
found. The effect of rest was to slowly restore the elasticity and 
finally raise it above the load that caused the previous deformation, 
but this occurred more slowly at room temperature than when the 
steel was warmed. While Howe's statements are not in entire 
agreement with the work of Bauschinger, his results appear 
consistent with the tests reported by the latter. 

Muir (44) found that the elastic recovery of overstrained steel 
was as marked after three or four minutes at 100° C (212° F) 
as in two weeks at room temperature and was impeded or entirely 
prevented at lower temperatures around 0° C (32° F). 

Howard (8) stated that "the effect of straining hot on the sub- 
sequent strength when tested cold appears to depend upon the 
magnitude of the straining force and the temperatiure when over- 
strained. There is a zone of temperatiure in which the effect of 
hot straining elevates the elastic limit above the applied stress 
and above the primitive value, and if the straining force approaches 
the present tensile strength there results a material elevation of 
that value when cooled. After exposure to higher temperatures 
a gradual loss occurs in both elastic limits and tensile strength^ 
and generally there follows a noticeable increase in the contrac- 



7o8 



Technologic Papers of the Bureau of Standards [voi.is 



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Steel at High Temperatures 



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yio Technologic Papers of the Bureau of Standards [Voi.i6 

tion of area." Howard found the zone of elevation of the elastic 
limit had apparently been passed when the steel was subjected 
to overstrain at about 395 to 425° C (740 to 800° F) and noted 
the importance of time between overload at high temperatures 
and subsequent test. 

Because of the extended field for research along these lines, as 
pointed out by Howard (8) , Jeffries (34) , and others, and because 
of the character and purpose of this investigation, the tests of 
overstrained steel made by the author were limited and under- 
taken with the view that the results might show some differences 
in deformational characteristics of steel above and at or below 
blue heat, for the most marked changes in tensile properties have 
been found in this temperature range. No generalizations are 
made from the data obtained in view of the omission of yield 
point determinations, but attention is drawn to certain of the 
observed effects shown in Table 8. 

At blue heat, 295° C (565° F), tensional stress in excess of 
the Hmit of proportionality raises this factor in a short time after 
release of load to a value approximating the stress producing the 
previous deformation, but when this stretching at blue heat is 
followed by cooling to room temperature and subsequent test the 
immediate effect of the "blue overstrain" is a lowering of the 
proportional hmit. This also appears to be the case at 465° C 
(870° F). 

Repeated overstrain at blue heat, whether at constant or 
increasing loads, raises the apparent Hmit of proportionality at 
this temperature in a Hke manner and to a similar degree. The 
magnitude of this increase may be considerable, as in the case of 
sample H 25, Table 8, where the proportional Hmit has been 
raised from 15 000 Ibs./in.^ to a Httle more than 56 000 Ibs./in.^, 
equivalent to 375 per cent. Such deformation does not, however, 
appear to modify the values of tensile strength, total elongation, 
or reduction of area when the specimen is finally broken. These 
results appear consistent with Bauschinger's deductions and the 
work of others mentioned, for a very rapid recovery from elastic 
overstrain and almost immediate elevation of the proportional 
limit would be expected at the temperature at which the stresses 
were applied. 

Heating to a temperature above Acj and thereafter cooling steel 
which has been overstrained at blue heat, 295° C (565° F), 
immediately restores the apparent elasticity of the metal at room 
temperattue. Such thermal treatment does not lower the Hmit 



French] Steel at High Temperatures 711 

of proportionality which in test at room temperature remains at 
a value approximating the previous overload at blueness. 

Metal which is imperfectiy elastic at room temperature because 
of previous overstrain at blue heat, 295° C (565° F), regains 
its elasticity after rest at room temperature. Steel subjected to 
overstrain at 465° C (870° F) behaves similarly, in so far as an 
increase above its original limit of proportionality is concerned. 
It is not qtdte clear from the several tests made if the immediate 
effect is to destroy the elastic properties, but such is probably the 
case. 

Rosenhain (45) reported in some cases at least the hardening of 
steel by plastic strain was unidirectional, and that a piece of steel 
which had attained an apparently raised proportional limit as a 
result of tensile overstrain was really not hardened in every way, 
for, if tested in compression, it was found that for stresses of that 
kind the apparent proportional limit had been lowered, so that 
the total range of elasticity from the limit of compression to that 
in tension had not been materially altered. 

In the search for differences in deformational characteristics 
with rise in temperature samples of marine boiler plate were sub- 
jected to stretching at several temperatures beyond the limit of 
proportionality and then tested in compression. The effect of 
such tensional overstrain at room temperature, blue heat and 
about 465°'C (870° F) is a lowering of the limit of proportionality 
in compression at room temperature. When the steel is allowed 
to rest, the samples stretched at blue heat or below show a gradual 
decrease with time of the compressive limit of proportionality 
which, when taken in conjunction with the rise in this factor in 
tension, indicates that the range of elasticity is not altered but 
merely shifted. Steel similarly stretched at 465° C (870° F) 
behaves differently, however, for with increase in time of rest the 
compressive limit of proportionality rises tmtil it approximates 
its original value, which in the cases shown in Figure 22 is reached 
after 71 hours. No interpretation of this latter effect can be at- 
tempted pending investigation of the effect of the magnitude of 
the tensional overstrain. There are, of coturse, many other ques- 
tions which arise in cormection with this subject which, as indi- 
cated heretofore, lead into a wide field for research but are not 
considered within the scope of this investigation. 



712 



Technologic Papers of the Bureau of Standards Woi. i6 



30 000 




to AO do 

Htmn afftr linsionaf Elastic 0*srsirain 



80 



Pig. 22. — Effect of time at room temperature on compressive proportional limit of marine 
boiler plate previously subjected to tensional elastic overstrain at various temperatures 





Stressed in tension 


Lb./in.2 


At °C 


A 
B 
C 


45000. 
45000 
30 000 


20 
29s 
S63 



Size of compression samples: H inch diameter, iK inches long. Gage length of i inch used. 

IV. EFFECT OF RATE OF LOADING ON THE TENSILE PROP- 
PERTIES OF STEEL AT VARIOUS TEMPERATURES 

Whether or not steel is susceptible to variations in rate of load- 
ing in those ranges of temperatures including superheater and 
boiler operation, crude oil distillation, nitrogen fixation, etc., is of 
decided interest both from practical and theoretical standpoints, 
especially as there are comparatively little definite data available 
in the Uterature. 

H. H. Campbell (47) reported the tensile properties of structural 
steel obtained under pulling speeds of 0.07 to 4.5 inches per minute. 
Both the yield point and tensile strength were shown to increase 
with rate of loading while the elongation and reduction of area re- 
mained practically constant. The fact that the last two factors 
which are independent of the accuracy of beam balance were not 
variable gives ground for thought concerning the causes for the 
susceptibility of strength and yield point to speed changes. When 
high pulling speeds are used, it becomes increasingly difficult for 
the operator properly to balance the beam of the testing machine,, 
and the tendency toward ' ' overbalancing ' ' often results in high 
values. 

A committee of the American Society for Testing Materials (46) 
has shown that the tensile properties of steels at room tempera- 



French] Stcel at High Temper atuYcs 713 

ture are independent of the rate of extension, at least within 
limits of commercial practice or covered by speeds of from i to 6 
inches per minute. 

Little exact information is available relative to effects of rate 
variations in application of stress 'at elevated temperatures. 
Hopkinson and Rogers (18) reported that as the temperature rose 
the stress-strain relations in steel underwent remarkable changes, 
which might best be expressed by saying that the variously called 
"time effect," or' "elastische nachwirkung," or "creeping," in- 
creased greatly with temperature. While such effects might be 
detected at ordinary temperatures, they attained a different order 
of magnitude at red heat (600° C) . 

The effect of ' ' creeping ' ' was found to make the determination 
of Young's Modulus a matter of some uncertainty, for the exten- 
sion of a bar stressed at 600° C varied 15 per- cent or more, de- 
pending upon the time of application of the load. For very short 
applications of the order of one or two seconds the strain produced 
approached a definite limiting value which, if used in determina- 
tion of the modulus, made it independent of the manner of load- 
ing and a physical constant. 

J. E. Howard (8) reported that the "rate of speed of testing 
which might modify the results somewhat with ductile material 
at atmospheric temperature had a very decided influence upon 
the apparent tenacity at high temperature." Steel containing 
0.81 per cent carbon was tested at the adopted speed of the series 
(5 to 10 minutes for rupture) and also under rapidly applied 
stresses (in which case the time employed to reach the maximum 
stress was from 2 to 8 seconds). Nearly the same strength was 
displayed whether slowly or rapidly fractured at ternperatures 
below about 315° C (600° F), this being a comparatively brittle 
metal at moderate temperatures; Above this temperature the 
apparent strength of the rapidly fractured specimens largely 
exceeded the strength of the others. The higher the temperature 
the wider apart in general were the results. An extreme illus- 
tration of this kind was furnished by a specimen tested at 766° C 
(1412° F) which when ruptured in two seconds showed a tensile 
strength of 62 000 lbs. /in. ^ as nearly as could be weighed, whereas 
at ordinary speed of testing a corresponding bar fractured at 
33 240 Ibs./in.^ 

Howard considered that the forces of cohesion tending to pre- 
vent rupture in a plane normal or oblique to the direction of the 
straining force and intermolecular friction developed during the 



714 Technologic Papers of the Bureau of Standards [Voi.ie 

flow of the metal were prominent or controlling elements in the 
explanation of the behavior of steel under the conditions outlined. 

More recently Rosenhain and Humfrey (29) have investigated 
the strength and fracture of soft steel at temperatures between 
600 and 1100° C (iioo and 2000° F). They found mechanical 
discontinuity in the thermal critical range and an increased 
tenacity with rising rates of extension in testing small samples in 
vacuum. 

In this investigation both increase in rates of extension over 
that adopted as standard (about 0.05 inch per minute average 
extension) and slow loading throughout the elastic range were 
studied and will be considered in order. 

1. RAPID LOADING 

(a) Apparatus Used — The ordinary method of determining 
the limit of proportionality at room temperature by measurement 
of deformation under successive increases in load, which has also 
been used in this investigation at higher temperatures, probably 
requires the simplest form of apparatus but is not sufficiently 
flexible to allow much variation in rate of extension without 
materially affecting the accuracy of the results obtained, as 
obviously it is more than difficult to read several continuously 
moving indicators simultaneously even with a number of observers. 
The original apparatus for determination of the proportional 
limit as described in the first part of this report was therefore 
modified in some essential details, so that instruments indicating 
stress and strain could together be rapidly and repeatedly photo- 
graphed by a motion-picture camera. The dials fastened to the 
frames shown in Figure 5 were turned so that their faces and a large 
load-indicating disk (Fig. 23) were practically in one plane. This 
latter was fastened to the uprights of the testing machine and by 
a system of pulleys connected to the screw operating the rider on 
the beam. One revolution of the disk, which was calibrated at 
50-pound intervals, is equivalent to 42 500-pound load. Heavy 
white twine treated with resin was used to operate the various 
pulleys and served very well without noticeable slippage. The 
purpose of the auxiHary load indicator is to bring the instruments 
measuring applied stress and resulting strain together in a small 
field in order that they may be simultaneously photographed 
and as large an image as possible obtained on the motion-picture 
film. 

For measuring deformation two geared dials with smallest 
direct reading of o.ooi inch were tried, as they were the only 



Technologic Papers of the Bureau of Standards, Vol. 16. 




Fig. 23. — Special apparatus used in determining tensile properties of steels 
at various temperatures tinder different rates of extension. [Auxiliary load 
indicator and motion-picture camera used in determining proportional limit) 







>* 


«'« 




^^^^^■^^ 


BBPu 




i\ 







Fig. 24. — Special apparatus used in determ,ining tensile properties of steels at 
various temperatures under different rates of extension. (Shows method used 
in operating auxiliary load indicator) 



Technologic Papers of the Bureau of Standards, Vol. 16. 




Fig. 25. — Enlargement of portion of fihn obtained with 
apparatus shown in figures 2j and 24 



French] Steel ttt High Temperatures 715 

type available but, owing to the relatively small distortion ob- 
tained at room and slightly elevated temperatm-es with moderate 
load increments, it was not possible to obtain the desired accuracy 
in strain measurements without reading the dials, in projecting 
the film, to one-tenth of the smallest division. These were there- 
fore discarded in favor of dials reading directly to 0.000 1 inch, 
but these latter instruments proved erratic at times, and it is felt 
that they were responsible for the greater part of the rejected 
runs where it was impossible to obtain any stress-strain diagrams 
from the dial movements. This difficulty can be overcome, how- 
ever, by the use of a special camera or lenses widening the field, 
so that dials with smallest direct reading of o.ooi inch, and esti- 
mated ten-thousandths can be used and read with ease from 
larger images on the films or preferably by the use of indicators 
free from gears. 

The frames carrying the dials were firmly attached to the speci- 
men at the 2-inch gage marks by means of two yokes, and the 
bar was then placed in the testing machine, heated to the desired 
temperature, and thermal equiUbrium established before loading 
began. The complete assembly of this apparatus, which is 
shown in Figures 23 and 24, was carried out in the same manner 
as described in Section II, 2. 

Load was next applied at any predetermined rate, and while 
the beam was at all times kept as nearly in balance as possible 
by the operator photographs were taken of the three constantly 
moving dials at the rate of about one a second. The loading was 
continuous until the bar was broken and no changes in gears or 
motor speed were made throughout the test. After development 
the film was projected on a screen where images of the dials were 
enlarged and as much time as was desired might be taken in 
obtaining individual readings. A simple projection device was 
used, and with this equipment it was found most convenient to 
obtain readings at a magnification of four times the original size. 
Under the conditions outlined above more photographs were 
obtained than required, but these served as a check on the accuracy 
of beam balance, which if not closely maintained resulted in serious 
deflections readily detected in the resulting stress-strain diagrams 
which were plotted in the usual manner. A portion of a typical 
film obtained in the determination of proportional limit by this 
method is shown enlarged in Figure 25, while a summary of re- 
sults of tests made at various temperatures under different rates 
of extension is given in Figures 26 and 27. 



7i6 



Technologic Papers of the Bureau of Standards [Voi. i6 




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French] 



Steel at High Temperatures 



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Technologic Papers of the Bureau of Standards \Voi. i6 



(b) Experimental ResuIvTs. — Based on tests made at room 
temperature, about 155° C (315° F), in the blue-heat range, 
295° C (565° F), and at 465° C (870° F) under varying rates of 
extension up to 1.6 inches per minute, the following conclusions 
may be drawn : 

1. The tensile properties of fire-box steel at temperatures up 
to and including the blue-heat range, 295° C (565° F), are inde- 
pendent of the rate of loading. 

2. At 465° G (870° F) the tensile strength appears to increase 
slowly with rate of loading, while ductility as measured by elonga- 
tion and reduction of area is practically constant. 

At this highest temperature the stress-strain diagrams obtained 
were not, on the whole, satisfactory, largely due in all probability 
to the low limit of proportionality which is reached soon after 
the "slack" is taken up in the geared dials, these being the only 
type available at the time the tests were made. The proportional 
limit rate of loading curve in Figure 27 is therefore tentative and 
shown as a dotted line. There is, however, a difference in behavior 
of the metal at or below blue heat and 465° C (870° F) as indicated 
by strength variations. 

2. SLOW LOADmG 

In addition to the foregoing tests samples were broken at about 
155° C (315° F), at blue heat, 295° C (565° F), and at 465^0 
(870° F) by increasing the applied stress very slowly while passing 
the proportional limit and somewhat beyond. The load was 
increased 100 pounds at five-minute intervals over a definite range 
at each temperattne, and subsequently the test specimen was 
broken at slow speed comparable to that adopted as the standard 

TABLE 9. — Effect of Slow Loading on the Tensile Properties of Fire-Box Boiler 
Plate at Different Temperatures (Series 1 Steel) 



Temperature 
of test 


Rate of loading 


Propor- 
tional 
limit 


Tensile 
strength 


Percent- 
age elon- 
gation in 
2 inches 


Percent- 
age 
reduction 
of area 


Remarks 


°C 

155 
156 


313 

313 

563 
563 

865 
865 


Adopted standard ' 

6| hours from 22 000 to 
47 000 lbs./in.2 

Adopted standard ' 

3i hours from 9000 to 
20 000 lbs./in.2 

Adopted standard > . . . . 
6 hours from 9000 to 
30 000 lbs./in.2 


Lbs./in.2 

26 600 


Lbs./in.2 
58 100 
64 350 

66 430 
60 000 

47 460 
33 600 


24.9 
22.8 

25.9 
36.0 

33.6 
42.0 


49.3 
45.9 

53.1 
59.2 

68.5 
78.4 


Average 3 tests 
Average 2 tests 


295 
295 


14 330 


Average 3 tests 


463 
463 


13 200 


Do. 









' Adopted standard averages about o.os inch per minute extension. 



Technologic Papers of the Bureau of Standards, Vol. 16. 




^J^ 



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Fig. 28. — Microphotographs of the fractures of hot-rolled fire box boiler 
plate at various temperatures. X 500 

a, Specimen A8; broken in tension at room temperature. 6, Specimen A_22; broken in 
tension at blue- heat (295° C). c. Specimen A5; broken in tension at 465° C. Etched 
with 2 per cent nitric acid in alcohol 



Technologic Papers of the Bureau of Standards, Vol. 16. 







Fig. 29. — Micro photographs of the fractures of cold-rolled marine boiler plate at 

various temperatures . X 500 

a. Specimen Fi; broken in tension at room temperature, b. Specimen F9-A; broken in tension 
at blue-heat (295° C). c, Specimen F9-B; broken in tension at blue-heat (295° C). (/.Specimen 
F14; broken in tension at 465° C. Etched with 2 per cent nitric acid in alcohol 



Technologic Papers of the Bureau of Standards, Vol. 16. 







■A sJki *^^-^. 




Fig. 30. — Micro photographs of the fractures of blue-rolled fire box boiler 
plate at various tetnperatures. X 500 

a, Specimen X7-1A; broken in tension at room temperature, b. Specimen Xy-iB; broken 
in tension at room temperature, c. Specimen X7-4; broken in tension at blue-heat 
(295° C). d. Specimen X7-7; broken in tension at 465° C. Etched with 2 per cent 
nitric acid in alcohol 



Technologic Papers of the Bureau of Standards, Vol. 16. 







a 



*■'.' , #^( 



■/ ..</: 




ir-'/^ .■ 




Fig. 31. — Micro photographs of the fractures of fire box boiler plate obtained 
at 46$° C. under different rates of loading. X 5^0 

a. Specimen K24; broken under standard rate of loading adopted. (About o.oj inch per 
minute free pulling speed of head of testing machine.) 6, Specimen K7-A; broken at 
0.5 inch per minute. (Free travel of head of testing machine.) c. Specimen K7-B; 
broken at o. 5 inch per minute. (Free travel of head of testing machine. ) d. Specimen 
L20 broken at 1.6 inch per minute. (Free travel of head of testing machine.) Etched 
with 2 per cent nitric acid in alcohol 



French] Stecl at High Temperatures 719 

throughout the investigation. Such slow loading raises the 
strength and decreases ductility at 155° C (315° F), but the steel 
exhibits decreased strength and higher ductility when so tested 
at blue heat, 295° C (565° F), or above than when broken in the 
ordinary manner (Table 9). 

V. MICROSCOPIC EXAMINATION 

Fractures of steel broken at various temperatures at different 
rates of loading and under varying conditions of mechanical work 
were examined under the microscope and were found to be 
generally transcrystalline. Minor differences appear under cer- 
tain conditions, such as, for example, the apparent tendency for 
the fracture to follow more deeply along the grain boundaries 
and particularly at the junctions of ferrite and pearlite when the 
steel is broken under very slowly increasing stress. In general, 
no marked differences in behavior were observed, as will be 
evident from microphotogriaphs shown in Figures 28 to 31, in- 
clusive. 

VI. DISCUSSION AND SUMMARY 
1. AMORPHOUS METAL THEORY 

Before concluding this report some of the observed phenomena 
will be briefly examined in the light of the amorphous theory of 
metals by which deformation is considered to take place along 
planes of easy slip in crystals. Along these gliding surfaces 
amorphous metal is formed which is likewise assumed to have 
temporary mobility and is followed by "setting" to produce 
extremely hard and rigid layers. This temporary mobility has 
been conveniently used in explaining the immediate loss of elas- 
ticity of overstrained metal and the subsequent elevation of the 
elastic limit with time. 

At the melting point the cohesion of the amorphous phase is 
zero and that of the crystalline a finite value. As the temper- 
ature is lowered the cohesion of the former increases more rapidly 
than that of the latter until at some temperature (called the 
equicohesive temperature) the cohesion-temperature curve of the 
amorphous phase intersects that of the crystalline. 

The difficulties of explaining certain observed phenomena and 
the basis for assuming a low-temperature allotrope in iron have 
recently been very completely stated by Jeffries (33) (34), but 
while such questions as why metals other than iron show no " blue- 



720 Technologic Papers of the Bureau of Standards [Voi.ie 

heat" range can not be satisfactorily answered by the amorphous 
metal theory or by differences in cohesion alone, the former is 
probably the most widely used and the hypothesis most generally 
adhered to. 

The differences in behavior of the limit of proportionality in 
compression with time in steel previously subjected to tensile 
overstrain above and at blue heat (respectively, 465 and 295° C) 
are not easily explained. The assumption of temporary mobility 
of the amorphous phase which has been used in explaining appa- 
rent loss of elasticity of iron overstrained at room temperature 
serves likewise for similar effects obtained at room temperature 
for steel subjected to overstrain at high temperatures. It is not 
clear why the unidirectionality of elastic recovery at room tempera- 
ture, and in these tests foiuid at blue heat, should be upset at the 
highest temperatin-e, 465° C (870° F), though it is suggested that 
the lowered cohesion of the crystalline phase permits more nearly 
indiscriminate slip along crystallographic planes instead of those 
most nearly normal to the straining force, so that after setting 
of the amorphous phase and coincident recovery of elasticity 
hardening is produced in all directions instead of along certain 
ones as found at blue heat and temperatiues below. Such indis- 
criminate slip might, of course, be favored under certain condi- 
tions of overstrain at blue heat and temperatures below, but, as 
previously indicated, insufficient data have been obtained to allow 
definite statements. That the hardening produced by blue and 
cold rolling is largely in the direction parallel to the axis of defor- 
mation is shown by the relatively small increases in strength 
obtained from such work on transverse samples. 

It is also conceivable that block movement of masses of atoms 
may break up into movements of smaller blocks if sufficient time 
is allowed. In other words, when a definite load in excess of the 
limit of proportionality is applied to steel at about 465° C (870° F) , 
deformation which at first takes place in block movement along 
planes of easy slip proceeds by further movement in these original 
blocks of smaller units resulting in creeping observed by Hopkin- 
son and Rogers (18), Howard (8), and others. This crystalline 
fragmentation would result in a general weakening of the cross 
section as the amorphous metal formed is weakened by the high 
temperature. As such an effect requires time and relatively large 
deformation, the slower the loading the lower the strength ex- 
pected. 



French] Steel at High Temperatures 721 

At 155° C (315° F) the cohesion of the amorphous phase is con- 
siderably greater than that of the crystalline and, in slow loading, 
time may be given for the "setting" of the amorphous metal 
formed. Hardening is therefore produced and results in higher 
strength. At both ordinary and rapid rates of loading insuffi- 
cient time is allowed for this setting and no marked changes in 
strength are observed. 

At blue heat, 295° C (565° F), the cohesion of the amorphous 
phase is less than at 155° C (315° F) but greater than at 465° C 
(870° F), which makes the behavior of steel in this range appar- 
ently anomalous, for in the range of maximum strength slow 
loading results in a decrease in this factor. 

While the amorphous metal theory is at this time the most 
widely used, it is not wholly adequate but is probably the best 
working hypothesis. Otur knowledge of the fimdamentals con- 
nected with changes in iron and steel below the thermal critical 
ranges will probably not be greatly enhanced by fiurther determi- 
nations of the mechanical properties alone. Such other methods 
as used by the Braggs * and others, while perhaps offering serious 
experimental difficulties, will without doubt more quickly lead to 
more truthful conceptions and explanations of observed phe- 
nomena. 

2. SUMMARY 

1. An apparatus has been devised for studying the changes in 
tensile properties of metals at various temperatures, iacluding 
determination of the limit of proportionality. A modified form 
of this equipment has been devised for studying the effects of 
variation in rapid rates of stress application on these properties, 
and this has been described. 

2. The proportional limit of low-carbon steel determined on 
longitudinal specimens does not decrease directly with first rise 
in temperature above that of the room, as has been so often 
reported, but is either maintained at about its room temperature 
value throughout a definite temperature range or increases before 
a marked drop in its value is observed. 

3. Changes in tensile strength and ductility of several grades 
of boiler plate from about 20 to about 465° C (70 to 870° F) have 
been determined. The general inflections in curves representing 
variations in these factors with rise in temperattu-e are the same 
for longitudinal and tranvserse specimens, and show maximum 

' W. H. Bragg and W. L. Bragg, X rays and crystal structure. 



722 Technologic Papers of the Bureau of Standards [Voi.z6 

strength between 250 and 300° C (480 to 570° F). Maximum 
ductility occiurs in longitudinal tests at 200 to 300° C (390 to 
570° F) and in transverse throughout a wider range from 150 to 
300° C (300 to 570° F). Above blue heat, 295° C (565° F), there 
is a marked drop in strength which is accompanied by decided 
inflections in curves showing elongations and reductions of area, 
indicative of a change in the character of the metal. 

4. The effect of moderate cold-rolling, which raises the elastic 
properties and to some extent the tensile strength and likewise 
lowers ductility at room temperature, is, in general, maintained 
throughout the range 20 to 465° C (70 to 870° F). However, at 
blue heat the increase in limit of proportionality due to previous 
cold work is greatly in excess of that observed at room temperature 
and is accompanied by an increased ratio, cold to hot rolled 
elongation. 

5 . If cold-rolled steel is heated for a short time at this tempera- 
ture (blue heat) and cooled, there results a decided elevation in 
the limit of proportionality with no material change in tensile 
strength or lowering of the ductility. This is of practical interest 
in production of such material as cold-drawn light-wall tubing 
often manufactrued under definite tensile requirements, where 
"bluing" subsequent to the last cold pass will result in improved 
tensile properties. 

6. The fact that blue-rolling is more effective than the same 
amount of cold-rolling in raising the strength of low-carbon steel 
and in decreasing ductility at room temperattue has been con- 
firmed. Six and one- quarter per cent reduction in thickness at 
blue heat produces about the same increase in strength at tem- 
peratures up to and including 295° C (565° F) (blue heat) as twice 
this cold reduction. Above blue heat the strength of the cold 
rolled steel is slightly in excess of that blue-rolled, though the 
general shape of the tensile properties temperature ciurves for both 
conditions mentioned is the same. 

7. Blue work (6.25 per cent reduction in plate thickness) is 
more effective in raising the limit of proportionality of low-carbon 
steel at temperatures below the blue-heat range than twice this 
work in the cold, but' at blue heat the increase in this factor is 
very much greater in the cold- worked metal. At higher tempera- 
tures the increase produced by both methods of working is approx- 
imately the same. 

8. In samples taken transversely the changes in tensile proper- 
ties throughout the range 20 to 465° C (70 to 870° F) resulting 



French] Stecl at High Temperatures 723 

from cold and likewise blue rolling are small compared to those 
observed in longitudinal tests. The similarity in the observed 
changes in tensile properties of low-carbon steel brought about by 
blue and cold deformation indicates that the character of the 
effects produced are similar but effected more rapidly at blue heat. 
While longitudinal and transverse tensile tests do not wholly 
define the character of the metal, sufficient evidence is presented 
to show the extreme susceptibility of steel to deformation in the 
blue-heat range, and for that reason alone such working should be 
avoided. A marked decrease in ductility results both at room 
and elevated temperatures. There is, however, little or no 
evidence to prove that a limited amount of blue work permanently 
injures the metal, for, as pointed out by Howe (38) and here 
substantiated by the author, restoration of ductility may be 
obtained by annealing. 

9. A few experiments relating to the effects of tensional elastic 
overstrain at various temperatures on the tensile properties of 
low-carbon steel at room temperature, blue heat, and 465° C 
(870° F) have been carried out. 

ID. A study of the effects of variations in rate of stress appli- 
cation from the adopted standard estimated at 0.05 inch average 
extension per minute to 1.6 inch per minute shows the tensile 
properties of steel to be independent of the rate of loading at 
temperatures up to and including blue heat. At 465° C (870° F) 
the tensile strength appears to increase slightly with increased 
rate of loading without noticeable change in ductility. 

11. Slow loading in the range about the proportional Hmit 
results in increased strength and decreased ductility at 155° C 
(315° F) and decreased strength and increased elongation and 
reduction of area at blue heat 295° C (565° F), and above 465° C 
(870° F). 

12. A brief discussion of the observed effects in the light of the 
amorphous metal theory has been given. 

3. ACKNOWLEDGMENTS 

The large number of tests reported in this investigation were 
made at intervals over a considerable period of time and required 
the aid of a number of assistants. Among these acknowledg- 
ments are due to the following: Donald S. Clements, temporarily 
laboratory assistant. Bureau of Standards, for aid in construction 
of the original apparatus and in making the first series of tests; 
A. Iv. Meyer, metallurgist, Lukens Steel Co., Coatesville, Pa., 



724 



Technologic Papers of the Bureau of Standards ivoi. 16 



for his cooperation, suggestions, and aid in testing several of the 
hot-rolled plates at various temperatures; A. T. Deny and assist- 
ants, for rolHng the various plates at room temperatures -and blue 
heat; Messrs. C. A. Newhouse and R. A. Bier, laboratory assistants, 
Bureau of Standards, who carried out a large number of the 
tests and rendered valuable aid in construction of the special 
apparatus used in rapid loading where records were obtained by 
the use of a motion-picture camera; R. Davis and M. Shannon, of 
the photographic laboratory, whose valuable suggestions in 
regard to this apparatus and operation of the motion-picture 
camera aided the author materially; T. G. Digges, laboratory 
assistant, and T. E. Hamill, laboratory aid, who assisted in 
the greater portion of tests of overstiained steel. 

VII. SELECTED BIBLIOGRAPHY 

1. PROPERTIES OF FERROUS ALLOYS AT ELEVATED TEMPERATURES 



No. 


Year 


1 


1837 


2 
3 
4 


1857 
1863 
1877 


5 


1878 


6 
7 


1880 
1890 


8 


1890 


9 


1893 


10 
11 
12 


1895 
1895 
1896 


13 


1900 


14 
15 
16 


1901 
1902 
1903 


17 


1904 


18 


1905 


19 


1905 


20 


1909 


21 
22 


1910 
1910 


23 
24 


1910 
1912 


23 


1912 


26 


1912 


27 


1913 


28 


1913 


29 


1913 



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Proc. Roy. Soc. of London, A, 83, p. 200. 
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Anon., The influence of increased temperature on the strength of certain metals and alloys, 

Metallurgical and Chem. Eng., 10, p. 160. 
Schultz, New researches with turbine materials for high temperature steam and gas tur- 

bhies. Die Turbine, 13, p. 14. 
P. Goerens and G. Hartel, Uber die zahigkeit des eisens bei verschiedenen temperaturen, 

Zs. anorg. Chem., 81, p. 130. 
W. Rosenhain and J. W. C. Humfrey, The tenacity deformation and fracture of soft steel 

at high temperatures, J. Iron and Steel Institute, 87, p. 238. 



French] 



Steel at High Temperatures 



725 



No. 



Year 



Reference 



30 


1914 


31 


1916 


32 


1917 


33 


1919 


34 


1920 


35 


1920 


36 


1921 



37 



1886 



38 
39 
40 
41 


1891 
1897 
1901 
1919 


42 


1879 


43 


1886 


44 


1900-1906 


45 


1915 


46 
47 


1906 
1907 



H. Perrine and O. B. Spencer, The strength of iron at varying temperatures, Columbia 

Univ. School of Mines Quarterly, 35, p. 194. 
O. Reinhold, The mechanical properties of steel at high temperatures, Ferrum, 13, p. 97, 

116, 129. 
F. A. Epps and E. O. Jones, Influence of hi^h temperatures upon elastic and tensile proper- 
ties of wrought iron. Metallurgical and Chem. En?., T7, p. 67, 
Zay Jeffries, Effect of temperature, deformation, and grain size on the mechanical properties 

of metals. Bull. Am. Inst. Min. Eng., 146, February. 
— ' Physical changes in iron and steel below the thermal critical range. Mining and 

Metallurgy, 158, sec. 20. 
A. E. White, Properties of iron and steel at high temperatures, J. Am. Steel Treaters' Soc, 

2, No. 10, September, p. 521. 
R. S. MacPherran, Comparative tests of steels at high temperatures, twenty-fourth annual 

meeting, A. S. T. M., June, 1921. 

2. ADDITIONAL REFERENCES 
(a) Cold and blue work 

S. F. Stromeyer, The injurious effect of a blue heat on steel and iron, Proc. Institution Civil 

Engr., 84, p. 14. 
H. M. Howe, The metallurgy of steel. 

A. Kurzwernhart, The influence of a blue heat, Stahl und Eisen, 41, p. 849. 
C. H. Ridsdale, The correct treatment of steel, J. Iron and Steel Institute, 1901, No. 2, p. 52. 
H. J. French, Manufacture and properties of light wall structural tubing, Bull. Am. Inst. 

Min. Engr., 153, p. 1855. 

(b) Elastic overstrain 

Charles Huston, The effect of continued and progressively increasing overstrain upon iron, 

J. Franklin Institute (January), p. 41. 
J. Bauschinger, Uber die veranderung der Elasticitatsgrenze und festikeit des eisens 

durch strecken und quetschen durch erwarmen und abkilhlen und oftmals wiederholte 

beanspriichung, Mittheilungen, Miinchen, 13. 
J. Muir, On the overstraining of iron, Phil. Trans. Roy. Soc. of London, A 193, p. l;Ibid., 

A 198, p. 1; Proc. Roy. Soc. of London, A 77, p. 277. 
W. Rosenhain, An introduction to physical metallurgy. 

(c) Rate of loading 

Report of Committee O on uniform speed in commercial testing, Proc. A. S. T. M., 6, p. 109. 
H. H. Campbell, The manufacture of iron and steel. 



Washington, March 2, 1922. 



Jt 



